mac80211: force use_short_slot=true for 5GHz

Normally 5GHz does not have a concept of long vs short slot time,
however the slot time that it ends up using is the same as for 2.4 GHZ
and use_short_slot == true
Because of that, it makes more sense to force use_short_slot = true
whenever 5 GHz is being used, so that this particular check does not
have to be in every single driver that uses this flag.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>